Insights into New Zealand Glacial Processes from studies of glacial geomorphology and sedimentology in Rakaia and other South Island Valleys

• Main Author: Hyatt, Olivia Marie
• Language: en
• Published: University of Canterbury. Geological Sciences 2010
• Subjects: glacial processes; glacial geomorphology; glacial sedimentology; Rakaia Valley; New Zealand
• Online Access: http://hdl.handle.net/10092/3699

This thesis investigates the assertion by many early and more recent New Zealand glacial workers, that the high catchment rainfall and low seasonality in New Zealand create unique glacial sedimentary and geomorphic processes. Specifically the thesis examines the nature of glacial sedimentology and geomorphology in South Island, New Zealand focussing on the Rakaia Valley, as most of the early studies that suggested a distinct New Zealand process environment were based on South Island, East Coast glacial valleys. The thesis provides insights into glacial processes operating at glacial termini of late Quaternary glaciers in this region. The primary findings are as follows: Glacial terminus landforms (moraines) and sediments are described in two eastern (Rakaia and Ashburton Lakes) and one western (Waiho) valley of South Island. There are three main types of landforms 1) outwash head, 2) push moraines and 3) ice-contact fans. Outwash heads and push moraines have been identified before in New Zealand, but ice-contact fans have not. The spatial relationships between the three landforms can be complex especially where there is a fluctuating glacier terminus. Outwash heads are the dominant landform, with ice-contact fans deposited at a stationary terminus with channelised meltwater and push moraines preserved during retreat accompanied with outwash head incision. Both ice-contact fans and push moraines are prone to reworking into the outwash head. Supraglacial material comprises a small cap on the moraines and is usually insignificant in this system. The nature of past glacier termini can be gained from detailed study of these three landform relationships and their sediment record. The dominance of glacifluvial processes at the glacier terminus is a reflection of the low seasonality, abundant catchment rainfall, coupled with a large sediment supply. Preservation and deposition of the push moraines and ice-contact fans are controlled by glacifluvial processes on an outwash head, which in turn are controlled by the mass balance of the glacier. Sedimentology, stratigraphy and facies architecture were examined in the lower Rakaia Valley and elsewhere. The main environments recorded by these sediments are largely proglacial lacustrine and fluvial including 1) outwash gravels, with deposition of a sequence of glacier-fed, Gilbert-type deltas deposited over buried ice at Bayfield Cliff, 2) lacustrine silts and sands, 3) sub-aqueous ice-contact fans, 4) sub-aqueous mass flow deposits, and 5) supraglacial melt out material. These glacilacustrine facies are widespread during both retreats and advances. Sub-aqueous deltas are the primary ice terminus form, in this mid-valley lacustrine setting, which record termini advance and retreats. Syn- and postdepositional deformation of lacustine facies are also common as a result of pushing and overriding from the fluctuating glacier termini. Buried ice is also widespread and many of these deposits display evidence of disruption of sedimentation by its meltout. This implies that stagnant tongues of ice were often buried by outwash and lacustrine sediments. From the sediments and geomorphology described in this thesis, two main glacier terminus settings in New Zealand valleys are apparent A) when the glacier terminus is on or abutting its outwash fan-head, or B) when the glacier terminus is within its trough. Both the geomorphic and edimentological findings allow a better understanding of New Zealand glacial chronologies. Firstly, the sedimentology permits the identification of many more advances and retreats than are recorded in surface sediments. At Rakaia Valley, facies record six significant advances and retreats and many more small oscillations over the last 200 000 years. The geomorphic understanding and high resolution mapping has identified many more ice termini in the valleys than were previously recognised and allow the insights into ice margin behaviour through time. This includes the changing location of outwash heads and glacial troughs, with a migration up-valley since the OIS 6 advance/s, in the Rakaia Valley. The glacier overran its outwash head to reach its LGM position, and subsequently retreated slowly over about 10,000 years, back to its outwash head. It then changed to a calving margin and continued retreating but with no terminal moraines preserved, only lateral features. The research in this thesis has contributed to greater understanding of the New Zealand glacial system. Although low seasonality and large volumes of meltwater do play a role, and equally important control in New Zealand valleys is that of tectonics in terms of delivering huge sediment supply. This sediment supply enables large outwash head and fans to accumulate, which allow large stable lakes to form during glacier recession. The data and interpretations from this thesis will underpin the development of a New Zealand glacial land system, of which other valleys such as the Himalayas have. This land system development is important for understanding the temperate, high sediment yield glacial environment end member.